Summary: The surface polarity of dimethylsiloxane-grafted silica particles (DMS/silica) has been studied as a function of surface coverage by measuring the UV/Vis spectra of coadsorbed solvatochromic probes. Fe(phen)2(CN)2 [cis-dicyano-bis-(1,10)-phenanthroline-iron(II), (1)] Michler's ketone [4,4′-bis(N,N-dimethylamino)-benzophenone, (2)], coumarine 153 (3) and 3-(4-amino-3-methylphenyl)-7-phenylbenzo[1,2b:4,5b′]difuran-2,6-dione (4) were used as solvatochromic surface polarity indicators. Kamlet–Taft's α (hydrogen bond acidity), β (hydrogen bond basicity), and π* (dipolarity/polarizability) parameters of the DMS/solid acid interfaces were calculated using different LSE relationships of data sets of the probes 1 and 2 or 3 (for α and π*) as well as 1 and 4 (for β). The values for α decrease significantly with increasing conversion of silanol groups with grafted DMS chains. Then, α asymptotically approachs a constant value when more than about 75% of the residual silanols on the surface are grafted with DMS when measured in 1,2-dichloroethane slurry. The π* values of DMS/silica solvent interfaces seems unaffected by the degree of surface functionalization. The β values of DMS and grafted DMS/silica are clearly lower than the values for α and π*.
Solvatochromic probes used for the study of the surface polarity of dimethylsiloxane-grafted silica particles.